General cellular polymer products have been commonly used industrially for a long time in order to make polymer products lightweight and save the required quantity of polymer. Of them, polystyrene foam products are representative and being used for a wide range of uses.
However, such general cellular polymer products have a cell size of 100 μm or so, so it is difficult to manufacture them into a continuous filament. Besides, they have a very low cell density of 106 cells/cm3, thus they are poor in touch and lightweight property and are difficult to acquire uniform physical properties.
To solve these problems, U.S. Pat. No. 5,866,053 and Pat. No. 6,051,174 disclose a method for making a microcellular extrusion materials in which a supercritical fluid such as CO2 is introduced into an extruder upon mixing and melting polymers in the extruder to prepare a single-phase solution of molten polymers and gas, and then the single-phase solution kept at a high pressure is extruded through a die to form a plurality of microcells by subjecting the single-phase solution to a rapid pressure drop.
The microcellular extrusion materials prepared by the above method is advantageous in that it provides cell sizes of less than 10 μm, which are smaller than the flaws preexisting within the polymers so that there occurs no decrease in the mechanical properties, and it provides high cell densities of 109 cells/cm3 or so, thus, the required amount of polymers can be saved. But, the above method is unsuitable for the manufacture of microcellular fibers since the molten polymer with a plurality of microcells are extruded into the air (at a room temperature) and slowly cooled down.
In other words, particularly, filaments, which are fibers of a continuous state, must undergo the process of making fine the extrusion materials spun from a spinneret through a very big deformation, the above method in which the molten polymer with a plurality of microcells are slowly cooled down after extrusion is unsuitable for a fiber manufacturing process, i.e., a filament spinning process.
Additionally, in case that the molten material prepared by the above method is melted and spun to make filaments for clothing such as polyamide filaments or polyester filaments, the melting strength of the spun filaments is low and thus a gas in the microcells flows out of the polymers immediately after the spinning (extruding), thus it is difficult to manufacture filaments (fibers) for clothing with high microcell densities.
To solve such a problem of an outflow of a gas in microcells, some methods for improving the melting strength of spun filaments by modifying polymers chemically have been attempted. But, in this case, there occurs a new problem such as a decrease of draw ratio in a drawing process, so this makes it difficult to manufacture microcellular fibers.
It is an object of the present invention to provide microcellular fibers for clothing which provide an excellent lightweight feeing and touch with microcells formed at a density of more than 107 cells/cm3.
It is another object of the present invention to effectively prevent the outflow of gas in microcells upon making microcellular fibers. It is another object of the present invention to effectively make microcellular fibers for clothing which provide an excellent lightweight feeling and touch with a plurality of microcells.